Sulfite oxidase is an enzyme located in the inter membranal space of the mitochondrion. The activity of this enzyme in the rat hepatic mitochondria declines during intoxication by carbon tetrachloride. The decline in activity is coincident with a disruption of the outer membrane of the mitochondrion and a decrease in the efficiency of oxidative phosphorylation. The restoration of mitochondrial function, marked by the return of respiratory control, and an intact outer membrane, precedes the return of normal levels of sulfite oxidase in the recovering animal. The processes responsible for the synthesis of sulfite oxidase and its insertion into a function location are under study to define their role in the repair processes of a cell damaged by acute chemical exposure.